A method for preparing a silicon-based silicon carbide DPF

Abstract

The present application relates to a kind of preparation methods of silicon-based silicon carbide DPF, using gas furnace to carry out oxygen firing, steps include by selecting specific particle size silicon carbide powder, silicon powder and appropriate amount of auxiliary, material is kneaded into plastic clay and is shaped into honeycomb ceramic monomer, and carries out drying, oxygen degassing and firing etc. After the monomer of firing is completed, high-pressure grouting mode is used, and the splicing material prepared from inorganic silica gel and silicon carbide powder is used to splice the monomer into different specifications of splicing body. After splicing body is dried and hardened, it is processed into cylinder with smaller diameter than finished product, and outer skin is made on its side. Finally, the required hole plugging effect is achieved by laser drilling combined with screen printing test hole plugging, and the preparation of silicon-based silicon carbide DPF product is completed. The preparation method of the present application can effectively prevent the oxidation of silicon powder during firing, improve the performance of silicon-based silicon carbide DPF, simplify the process flow, reduce the preparation cost, and be easy to realize industrial production.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of preparation of silicon carbide DPF, more particularly to a preparation method of silicon-based silicon carbide DPF. BACKGROUND

[0002] In recent years, with the increasingly serious environmental pollution problem, especially the particulate matter (PM) contained in the exhaust gas of diesel engine has caused serious influence on the environment and human health. Therefore, effective control and treatment of the particulate matter emitted by diesel engine has become the focus of current research and technical development. Among them, diesel particulate filter (DPF) is considered as a key technology for effectively reducing the particulate matter emission of diesel engine exhaust gas.

[0003] At present, in the field of DPF, silicon-based silicon carbide SiC material has been widely concerned and applied due to its excellent thermal performance, chemical stability, high thermal conductivity, high permeability and many other advantages. However, in the preparation process of traditional silicon-based silicon carbide DPF, the existence of metallic silicon powder is easy to cause oxidation to silicon dioxide, which further affects its performance. Therefore, how to prevent the oxidation of metallic silicon powder during the preparation process is a major challenge at present.

[0004] In order to solve this problem, previous studies have adopted vacuum or inert gas protection method to prevent the oxidation of metallic silicon powder, but this method has the disadvantages of complex equipment and high cost. Therefore, a relatively simple and economical method is urgently needed to solve this problem in industrial production. SUMMARY

[0005] In order to solve the problems existing in the prior art, a preparation method of silicon-based silicon carbide DPF is provided, and the specific scheme is as follows:

[0006] A preparation method of silicon-based silicon carbide DPF, which is oxygen-burned by a gas furnace, comprising the following steps:

[0007] S1: selecting refined silicon carbide powder, the average particle size of the silicon carbide powder is 25-35 μm, and the fine powder D10≥3 μm is removed; selecting metallic silicon powder with an average particle size of ≤5 μm, the addition amount is ≤8%; adding high plasticity ultra-fine washed kaolin, 0.5-2.5% of boride, 5-15% of organic powder pore-forming agent;

[0008] S2: After the above material is fully stirred and kneaded with the organic binder, the dispersant, lubricant and mixing water are added to knead the material into plastic clay, the plastic clay is formed into honeycomb ceramic monomers by using a high-pressure vacuum extruder, drying is performed, and the honeycomb ceramic monomers are hardened; after the hardening, the monomers are loaded into open-type saggar, aerobic degumming is performed according to a certain temperature rising curve, and the organic binder and pore-forming agent in the interior of the green body are effectively decomposed and removed during the degumming stage. The complete removal of the organic binder and pore-forming agent not only ensures the smooth progress of the subsequent sintering stage and the stable structure of the product, but also reduces the influence of the organic binder and pore-forming agent on the performance of the silicon-based silicon carbide DPF. After the degumming treatment is completed, the degummed product is sintered again;

[0009] S3: The residual carbon is uniformly arranged around the monomers after the degumming is completed, a sealing cover is added to each saggar, and the monomers after the degumming are sintered according to a certain sintering curve; the sintering method of the saggar with the cover is adopted, and the possibility of the product contacting with oxygen during the sintering process is reduced by covering the saggar. During the sintering, the residual carbon of 0.3-0.8% is reserved in the interior of the green body. During the sintering process, the residual carbon can react with the residual oxygen in the saggar, thereby consuming the residual oxygen in the saggar and reducing the risk of oxidation of the silicon powder. At the same time, the temperature rising rate and sintering temperature are controlled during the sintering process, so as to ensure that the silicon powder is not oxidized. Through the optimization design of the two steps, the oxidation of the silicon powder during the sintering process can be effectively prevented, and the silicon-based silicon carbide DPF product with superior performance such as thermal expansion coefficient and thermal conductivity coefficient can be obtained.

[0010] S4: After the sintering of the monomers is completed, the monomers are spliced into spliced bodies of different specifications by using high-pressure grouting and using the splicing material prepared from inorganic silica gel and silicon carbide powder; after the spliced bodies are hardened by hot air drying, the spliced bodies are processed into cylinders with a diameter smaller than that of the finished product by using an external grinding machine;

[0011] S5: The outer skin is made on the side surface of the cylinder by using an automatic skin grafting machine; after the outer skin is dried, the straight-through carrier is plugged according to the requirements by using a laser drilling machine combined with a silk screen printing plugging machine, and the preparation of the product is completed.

[0012] In the step S1, the average particle size of the added silicon powder is re-optimized, so as to ensure that the silicon powder can be better filled between the silicon carbide powder particles, thereby reducing the addition amount of the silicon powder as much as possible.

[0013] In order to protect the silicon powder and prevent it from being oxidized during the sintering process, high-plasticity ultra-fine washed kaolin is added, the addition ratio of the high-plasticity ultra-fine washed kaolin is the same as that of the silicon powder, and the average particle size of the high-plasticity ultra-fine washed kaolin is 0.1-0.2 times that of the silicon powder. Through such design, the kaolin powder can form a wrapping layer on the surface of the silicon powder during the kneading and clay preparation process, thereby preventing the silicon powder from being oxidized.

[0014] To further prevent the oxidation of the metal silicon, borides are added, the borides being boron nitride or titanium diboride, the melting point of the boron nitride or titanium diboride being high in the presence of oxygen, and the stability of the product under high temperature conditions can be improved. Based on the excellent oxidation resistance and thermal stability of the aforementioned borides, the metal silicon powder is protected from oxidation during the sintering process, thereby ensuring that the performance indicators of the silicon-based silicon carbide DPF, such as the thermal conductivity and the thermal expansion coefficient, meet the requirements.

[0015] Since the DPF is a high-porosity product, the accumulation of silicon carbide powder alone cannot meet the requirement of high porosity, and therefore an organic powder pore-forming agent needs to be added to the formula to meet the final product porosity ≥ 38%.

[0016] Further, the fine powder D10 ≥ 5 μm is removed in S1.

[0017] Further, the addition amount of the metal silicon powder in S1 is ≤ 5%.

[0018] Further, the addition ratio of the borides is 0.5-1.5%.

[0019] Further, the organic powder pore-forming agent is one or a mixture of several of polyethylene powder, polypropylene powder, microcrystalline wax, polyformaldehyde foam and starch.

[0020] Further, the organic binder is one or a mixture of several of polyvinyl alcohol, hydroxypropyl methylcellulose and hydroxyethyl methylcellulose.

[0021] Further, the dispersant in S2 is one or a mixture of several of polyacrylate, polymethacrylate, polyphosphate sodium, ethanolamine and cellulose acetate.

[0022] Further, the lubricant in S2 is one or a mixture of both of low polymerization degree PE wax and polyethylene wax.

[0023] Further, the retention amount of the residual carbon in S3 is 0.3-0.5%.

[0024] Further, for the temperature curve in S2, it can be selected to gradually rise from room temperature to 300-400°C, and the temperature rising rate can be controlled at 1-5°C / min during the process. After reaching the set temperature, the temperature is maintained for 1-4 hours to ensure that the organic binder and the pore-forming agent inside the green body are effectively decomposed and removed. After the degassing treatment is completed, the degassed product is sintered.

[0025] Further, in S3, the heating rate and the sintering temperature need to be controlled during sintering. The heating rate can be controlled at 1-5℃ / min, and the final sintering temperature can be set at 1200-1500℃, and then the temperature is maintained for 2-8 hours to ensure that the metal silicon powder is not oxidized and the product performance is stable. Through the optimization design of the two steps, the metal silicon powder can be effectively prevented from being oxidized during sintering, so that the silicon-based silicon carbide DPF product with excellent performance such as thermal expansion coefficient and thermal conductivity coefficient can be obtained.

[0026] Advantages:

[0027] The application provides a preparation method of a silicon-based silicon carbide DPF, which has the following advantages:

[0028] (1) By selecting silicon carbide powder and metal silicon powder with appropriate particle size and adding high-plasticity ultra-fine washed kaolin, the mixing uniformity of the silicon carbide and the metal silicon powder can be improved, so that the pore distribution, thermal conductivity and thermal stability of the silicon-based silicon carbide DPF can be improved.

[0029] (2) By adding borides during preparation, the metal silicon powder can be effectively prevented from being oxidized during sintering, so that the performance of the silicon-based silicon carbide DPF can be improved. At the same time, by adjusting the addition ratio of the borides, the thermal stability of the silicon-based silicon carbide DPF can be optimized.

[0030] (3) By adding an appropriate amount of organic powder pore-forming agent, the silicon-based silicon carbide DPF can be provided with good porosity, so that the dust filtration performance thereof can be improved.

[0031] (4) By setting reasonable degassing treatment and sintering process parameters, the silicon-based silicon carbide DPF can be ensured to have good mechanical properties and dust filtration efficiency, so that the service life of the product can be improved.

[0032] (5) By using the method of sintering with a cover on the sagger, the residual carbon in the green body can be effectively utilized to consume the residual oxygen in the sagger, so that the metal silicon powder can be further prevented from being oxidized during sintering, and the prepared silicon-based silicon carbide DPF has excellent comprehensive performance.

[0033] (6) The preparation method of the silicon-based silicon carbide DPF provided by the application is simple in process, does not need additional inert gas protection or vacuum equipment, reduces the preparation cost, is easy to realize industrialized production, and has high practical value. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is a micro scanning electron microscope of the product prepared in the example Figure 1 .

[0035] Figure 2 is a micro scanning electron microscope of the product prepared in the example Figure 2 .

[0036] Figure 3 Microscopic scanning electron microscope of the product prepared in the example Figure 3 . DETAILED DESCRIPTION

[0037] In order to deepen the understanding of the present application, the present application will be further described in conjunction with the examples below, which are only used to explain the present application and do not constitute a limitation on the protection scope of the present application.

[0038] Example 1:

[0039] A preparation method of a silicon-based silicon carbide DPF, which is oxygen-burned by a gas furnace, comprising the following steps:

[0040] Step one: select silicon carbide powder with an average particle size of 30 μm, and remove fine powder D10 of 3 μm; select metal silicon powder with an average particle size of 4 μm, and add 5%; add high-plasticity ultra-fine water-washed kaolin, 1% boron nitride, and 10% polyethylene powder pore-forming agent.

[0041] Step two: after the above materials are fully stirred and kneaded with polyvinyl alcohol, add polyacrylate dispersant, polyethylene wax lubricant, and mixing water to knead the materials into plastic mud, use a high-pressure vacuum extruder to shape the plastic mud into a honeycomb ceramic monomer; dry in a microwave oven to a water content of <2%, and harden the honeycomb ceramic monomer; after hardening, put the monomer into an open-type saggar, and perform oxygen-burning of the glue according to a certain temperature rising curve (gradually rising from room temperature to 320℃, control the temperature rising rate at 2℃ / min, after reaching the set temperature, maintain the set temperature for 2).

[0042] Step three: evenly arrange 2g of residual carbon around the monomer after glue removal, add a sealing cover to each saggar, and perform burning of the glue-removed monomer according to a certain burning curve (control the temperature rising rate at 4℃ / min, and set the final burning temperature at 1300℃, maintain this temperature for 4h).

[0043] Step four: after the monomer is burned, use high-pressure grouting to splice the monomer into a 4*4 splicing body using splicing material prepared from inorganic silica gel and silicon carbide powder; after the splicing body is hardened by hot air drying, use an external grinding machine to process the splicing body into a cylinder with a diameter less than 4mm of the finished product.

[0044] Step five: use an automatic skin grafting machine to scrape and coat a layer of 1mm-thick mud on the side of the cylinder to make an outer skin for the processed splicing body; after the outer skin is dried, use a laser drilling machine combined with a silk screen printing hole blocking machine to block the holes of the straight-through carrier according to the requirements, and complete the preparation of the product.

[0045] Example 2:

[0046] A preparation method of silicon-based silicon carbide DPF, which is oxygen-burned by a gas furnace, comprises the following steps:

[0047] Step one: select silicon carbide powder with an average particle size of 30 μm, and remove fine powder with D10 of 5 μm; select metal silicon powder with an average particle size of 4 μm, and add 4%; add high-plasticity ultra-fine washed kaolin, 1% boron nitride, and 10% polyethylene powder pore-forming agent.

[0048] Step two: after the above materials are fully stirred and kneaded with polyvinyl alcohol, polyacrylate dispersant, polyethylene wax lubricant, and mixing water, the materials are kneaded into plastic mud, the plastic mud is formed into honeycomb ceramic monomers by using a high-pressure vacuum extruder, and the honeycomb ceramic monomers are hardened by drying in a microwave oven until the water content is less than 2%; after the hardening, the monomers are loaded into open-type saggar, and oxygen-burning is performed according to a certain temperature rising curve (gradually rising from room temperature to 320℃, the temperature rising rate is controlled to be 2℃ / min, after reaching the set temperature, the set temperature is maintained for 2h).

[0049] Step three: 5g of residual carbon is uniformly arranged around the monomers after the burning, a sealing cover is added to each saggar, and the monomers after the burning are fired according to a certain firing curve (the temperature rising rate is controlled to be 4℃ / min, and the final firing temperature is set to be 1300℃, and the temperature is maintained for 4h).

[0050] Step four: after the monomers are fired, the monomers are spliced into a 5*5 splicing body by using high-pressure grouting and using splicing material prepared from inorganic silica gel and silicon carbide powder; after the splicing body is hardened by hot air drying, the splicing body is processed into a cylinder with a diameter less than 4mm of the finished product by using an external grinding machine.

[0051] Step five: a layer of 2mm-thick mud is scraped on the side of the cylinder by using an automatic skin grafting machine, a layer of skin is made on the processed splicing body; after the skin is dried, the straight-through carrier is plugged according to requirements by using a laser drilling machine combined with a silk screen printing plugging machine, and the preparation of the product is completed.

[0052] Example 3:

[0053] A preparation method of silicon-based silicon carbide DPF, which is oxygen-burned by a gas furnace, comprises the following steps:

[0054] Step one: select silicon carbide powder with an average particle size of 30 μm, and remove fine powder with D10 of 3 μm; select metal silicon powder with an average particle size of 4 μm, and add 5%; add high-plasticity ultra-fine washed kaolin, 1% titanium diboride, and 10% polyethylene powder pore-forming agent.

[0055] Step two: After the above materials are kneaded with polyvinyl alcohol, polyacrylate dispersant, polyethylene wax lubricant and mixed water, the materials are kneaded into plastic mud, and the plastic mud is formed into honeycomb ceramic monomers by using a high-pressure vacuum extruder; the monomers are dried in a microwave oven to a water content of less than 2%, and the monomers are hardened; after hardening, the monomers are loaded into an open type saggar, and oxygen assisted resin removal is performed according to a certain temperature rising curve (gradually rising from room temperature to 320℃, the temperature rising rate is controlled at 2℃ / min, after reaching the set temperature, the set temperature is maintained for 2h).

[0056] Step three: After the completion of the resin removal of the monomers, 3g of residual carbon is uniformly arranged around the monomers, a sealing cover is added to each saggar, and the monomers after resin removal are fired according to a certain firing curve (the temperature rising rate is controlled at 4℃ / min, the final firing temperature is set at 1300℃, and the temperature is maintained for 4h).

[0057] Step four: After the completion of the monomer firing, the monomers are spliced into a 6*6 splicing body by using high-pressure grouting and using splicing material prepared from inorganic silica gel and silicon carbide powder; after the splicing body is hardened by hot air drying, the splicing body is processed into a cylinder with a diameter of less than 4mm of the finished product by using an external grinding.

[0058] Step five: A layer of 1mm thick mud is scraped on the side of the cylinder by using an automatic skin grafting machine, and a layer of skin is made for the processed splicing body; after the skin is dried, the straight-through carrier is plugged according to the requirements by using a laser drilling machine combined with a silk screen printing plugging hole machine, and the preparation of the product is completed.

[0059] Example 4:

[0060] A preparation method of a silicon-based silicon carbide DPF, which is fired by using a gas furnace, comprises the following steps:

[0061] Step one: silicon carbide powder with an average particle size of 30μm is selected, and the fine powder D10 is 3μm after removing the fine powder; metal silicon powder with an average particle size of 4μm is selected, and the addition amount is 5%; high plasticity ultra-fine water-washed kaolin, 1% boron nitride and 10% starch pore-forming agent are added.

[0062] Step two: After the above materials are kneaded with polyvinyl alcohol, polyacrylate dispersant, polyethylene wax lubricant and mixed water, the materials are kneaded into plastic mud, and the plastic mud is formed into honeycomb ceramic monomers by using a high-pressure vacuum extruder; the monomers are dried in a microwave oven to a water content of less than 2%, and the monomers are hardened; after hardening, the monomers are loaded into an open type saggar, and oxygen assisted resin removal is performed according to a certain temperature rising curve (gradually rising from room temperature to 320℃, the temperature rising rate is controlled at 2℃ / min, after reaching the set temperature, the set temperature is maintained for 2h).

[0063] Step three: After the completion of the glue of the monomer, the residual carbon of 2g is evenly arranged around the monomer, a sealing cover is added to each of the sagger, and the monomer after the glue is removed is fired according to a certain firing curve (the heating rate is controlled at 4 ℃ / min, and the final firing temperature is set at 1300 ℃, and the temperature is maintained for 4h).

[0064] Step four: After the completion of the monomer firing, the monomer is spliced into a 5*5 splicing body by using high-pressure grouting and using the splicing material prepared by inorganic silica gel and silicon carbide powder; after the splicing body is dried and hardened by hot air, the splicing body is processed into a cylinder with a diameter less than 4mm of the finished product by using an external grinding.

[0065] Step five: A layer of 1mm thick mud is scraped on the side of the cylinder by using an automatic skin grafting machine, and a layer of skin is made for the processed splicing body; after the skin is dried, the product is prepared by using a laser drilling machine combined with a silk screen printing hole sealing machine to seal the holes according to the requirements.

[0066] Example 5:

[0067] A preparation method of a silicon-based silicon carbide DPF, which is fired by using a gas furnace, comprises the following steps:

[0068] Step one: silicon carbide powder with an average particle size of 30μm is selected, and the fine powder D10 is 3μm after removing the fine powder; metal silicon powder with an average particle size of 4μm is selected, and the addition amount is 5%; high plasticity ultra-fine water-washed kaolin, 1% boron nitride, and 10% polyethylene powder pore-forming agent are added.

[0069] Step two: after the above materials and hydroxyethyl methyl cellulose are fully stirred and kneaded, polyacrylate dispersant, polyethylene wax lubricant, and blending water are added to knead the materials into plastic mud, the plastic mud is formed into a honeycomb ceramic monomer by using a high-pressure vacuum extruder; the honeycomb ceramic monomer is dried in a microwave oven to less than 2% of water content, and the monomer is hardened; after the hardening, the monomer is loaded into an open sagger, and oxygen is removed according to a certain temperature rising curve (the temperature is gradually increased from room temperature to 320℃, the temperature rising rate is controlled at 2 ℃ / min, and the temperature is maintained at the set temperature for 2h).

[0070] Step three: 3g of residual carbon is evenly arranged around the monomer after the completion of the glue, a sealing cover is added to each of the sagger, and the monomer after the glue is removed is fired according to a certain firing curve (the heating rate is controlled at 4 ℃ / min, and the final firing temperature is set at 1300 ℃, and the temperature is maintained for 4h).

[0071] Step four: after the completion of the monomer firing, the monomer is spliced into a 6*6 splicing body by using high-pressure grouting and using the splicing material prepared by inorganic silica gel and silicon carbide powder; after the splicing body is dried and hardened by hot air, the splicing body is processed into a cylinder with a diameter less than 4mm of the finished product by using an external grinding.

[0072] Step five: adopt automatic skin grafting machine to scrape and coat a layer of 1mm thick mud on the side of the cylinder, make a layer of skin for the processed splicing body; after the skin is dried, use laser punching machine combined with silk screen printing hole blocking machine to block holes according to requirements for the straight-through carrier, complete the preparation of the product.

[0073] The product prepared by the above preparation method is characterized by microstructure, and the results are shown in Figs. Figure 1 、 2 3, from which it can be seen that silicon carbide is not oxidized to silicon dioxide, which shows that the present preparation method achieves the purpose of preventing the oxidation of silicon powder without using vacuum or inert gas protection.

[0074] As a further improvement, the above-mentioned is only a preferred embodiment of the present application, and is not used to limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method for preparing silicon-based silicon carbide DPF, characterized in that, The aerobic firing process using a gas-fired furnace includes the following steps: S1: Select refined silicon carbide powder, wherein the average particle size of the silicon carbide powder is 25~35μm, and the fine powder D10 after removing micro powder is ≥3μm; select metallic silicon powder with an average particle size ≤5μm, and add ≤8%; add high plasticity ultrafine water-washed kaolin, 0.5~2.5% boride, and 5~15% organic powder pore-forming agent; S2: After thoroughly mixing and kneading the above materials with the organic binder, add the dispersant, lubricant and water to knead the materials into a plastic mud. Use a high-pressure vacuum extruder to form the plastic mud into honeycomb ceramic monomers; dry to harden the honeycomb ceramic monomers; after hardening, put the monomers into an open sagger and perform aerobic debinding according to a certain heating curve. S3: Evenly distribute residual char around the monomer after debinding, add a sealed lid to each sagger, and fire the monomer after debinding according to a certain firing curve. S4: After the monomers are fired, they are spliced ​​into splice bodies of different specifications using a splicing material made of inorganic silicone and silicon carbide powder through high-pressure grouting. After the splice bodies are dried and hardened by hot air, they are processed into cylinders with a diameter smaller than that of the finished product using an external cylindrical grinder. S5: An automatic skin grafting machine is used to create an outer skin on the side of the cylinder; after the outer skin dries, a laser drilling machine combined with a screen printing test plugging machine is used to plug the holes in the through carrier according to the requirements to complete the product preparation. In S1, the fine powder with D10 ≥ 5 μm after removing micro powder is removed, the amount of added metallic silicon powder is ≤ 5%, the addition ratio of boride is 0.5~1.5%, and the addition ratio of the high plasticity ultrafine water-washed kaolin is the same as that of metallic silicon powder, with an average particle size of 0.1~0.2 times that of metallic silicon powder. The boride is one or a mixture of two of boron nitride or titanium diboride.

2. The method for preparing a silicon-based silicon carbide DPF according to claim 1, characterized in that, The organic powder pore-forming agent is one or a mixture of several of the following: polyethylene powder, polypropylene powder, microcrystalline wax, polyoxymethylene foam, and starch.

3. The method for preparing a silicon-based silicon carbide DPF according to claim 1, characterized in that, The organic binder is one or a mixture of several of polyvinyl alcohol, hydroxypropyl methylcellulose, and hydroxyethyl methylcellulose.

4. The method for preparing a silicon-based silicon carbide DPF according to claim 1, characterized in that, The amount of carbon residue retained in S3 is 0.3~0.5%.

5. The method for preparing a silicon-based silicon carbide DPF according to claim 1, characterized in that, The process parameters in S2 are as follows: gradually increase the temperature from room temperature to 300~400℃, control the heating rate at 1~5℃ / min, and maintain the set temperature for 1~4 hours after reaching the set temperature.

6. The method for preparing a silicon-based silicon carbide DPF according to claim 1, characterized in that, The dispersant in S2 is one or a mixture of several of the following: polyacrylate, polymethacrylate, sodium polyphosphate, ethanolamine, and cellulose acetate.

7. The method for preparing a silicon-based silicon carbide DPF according to claim 1, characterized in that, The lubricant in S2 is one or a mixture of two of low-polymerization degree PE wax and polyethylene wax.

8. The method for preparing a silicon-based silicon carbide DPF according to claim 1, characterized in that, During the firing process of S3, the heating rate is controlled at 1~5℃ / min, and the final firing temperature is set at 1200~1500℃ and maintained at this temperature for 2~8 hours.

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